The present invention concerns systems for finding the direction of a received electromagnetic signal. It is directed specifically to such devices employing circular arrays of antenna elements.
A powerful device for use in finding the direction of an incoming electromagnetic signal is the two-dimensional compressive receiver. A two-dimensional compressive receiver employs a dispersive sonic delay line employing a number of input transducers arrayed along one edge and a number of output transducers arrayed along the opposite edge. If the signals applied to the input transducers originate in a linear array of antenna elements sensing a single plane wave, the phase relationship among the input transducers will be a linear function of element position and plane-wave bearing angle. If the bearing of the plane wave is perpendicular to the axis of the array, all of the transducer signals will be in phase. As the bearing angle increases, so does the phase difference between successive antenna elements. In the delay line, the input transducers set up interference patterns that result in constructive interference and thus a maximum at a point on the output edge that depends on the phase advance between successive input transducers of the delay line. Thus, the direction from which the signal is received is indicated by the position of the output transducer with the greatest signal.
The magnitude of the phase advance can be thought of as a spatial frequency. Just as temporal frequency is the rate of change of phase with respect to time, spatial frequency is the rate of change of phase with respect to position. The point on the output edge of the delay line where the maximum occurs is dependent on the spatial frequency at the input edge of the delay line. Thus, the delay line can be thought of as a device for performing a spatial Fourier transformation.
In addition to the spatial transformation, the two-dimensional compressive receiver also performs a temporal Fourier transformation. The compressive-receiver delay line is characterized by a delay that is a linear function of frequency. The input signals are mixed with a signal from a swept local oscillator whose frequency as a function of time corresponds to the delay-line relationship of delay to frequency in such a way that portions of an input signal at a given frequency occurring late in a local-oscillator sweep cause delay-line signals that propagate more rapidly than those caused by earlier input-signal portions at the same frequency. Thus, an input signal of a given frequency that lasts throughout the sweep of the local oscillator causes an output signal that is a very short pulse. The time at which the pulse occurs is an indication of the frequency of the signal that gave rise to it. Thus, the two-dimensional compressive receiver performs a two-dimensional Fourier transformation from time and position to temporal and spatial frequency.
In the discussion so far, it has been assumed that the antenna elements for sensing the incoming signal are disposed in a linear array. However, circular arrays are sometimes preferable, and it would be desirable to obtain the benefits of the two-dimensional compressive receiver with such arrays.
An arrangement has been proposed by Victor A. Misek for employing a four-element circular array with the two-dimensional compressive receiver, or, indeed, with any device for performing a spatial Fourier transformation, regardless of whether it simultaneously performs a temporal Fourier transformation. According to the Misek arrangement, the difference between the signals from two diametrically opposed elements is phase shifted by 90.degree. and added to that from the other two elements, which is not phase shifted. The resultant signal, whose phase angle equals the plane-wave bearing angle, is applied to one input transducer of a two-input-transducer sonic delay line. The other input transducer receives the sum of all the element signals, which sum signal has a phase that is substantially independent of bearing angle. Four transducers on the output edge of the delay line provide a sample of the resultant interference pattern at the output edge of the delay line, and the resultant signals are detected, fed through logarithmic amplifiers, and processed by an algorithm for determining the bearing angle from the outputs.
The bearing-angle resolution of such a device is determined by the input aperture--that is, by the distance between the transducer elements that define the ends of the pattern set up on the input edge. A larger aperture results in greater bearing-angle resolution. Unfortunately, increased separation between the transducers can result in grating lobes--that is, spurious peaks at misleading positions on the output edge of the transducer. Accordingly, the bearing-angle resolution obtainable with this type of system is limited.
An object of the present invention is to employ the two-dimensional compressive receiver--or some other spatial-Fourier-transform device--with a circular array in a manner that, in principle, permits an arbitrarily high bearing-angle resolution.